447 



LECTURE XXXVIIL 



ON VISION. 



The medium of communication, by which we become acquainted with all 

 the objects that we have been lately considering, is the eye; an organ that 

 exhibits, to an attentive observer, an arrangement of various substances, so 

 correctly and delicately adapted to the purposes of the sense of vision, that 

 we cannot help admiring, at every step, the wisdom by which each part is 

 adjusted to the rest, and made to conspire in effects, so remote from what 

 the mere external appearance promises, tlut we have only been able to un- 

 derstand, by means of a laborious investigation, the nature and operations 

 of this wonderful structure, while its whole mechanism still remains far be- 

 yond all rivalship of human art.. 



The eye is an irregular spheroid, not very widely differing from a sphere ; 

 it is principally composed of transparent substances, of various refractive, 

 densities, calculated to collect the rays of light, which diverge from each 

 point of an object, to. a focus on its. posterior surface, which is capable of 

 transmitting to the mind the impression of the colour and intensity of the; 

 light, together with a distinction of the situation of the focal point, as de.- 

 termined by the angular place of the object. (Plate XXX. Fig. 436.), 



The first refraction happens at the surface of the cornea, or that transparent 

 eoat which projects forwards from the ball of the eye; but the cornea, 

 being very nearly of equable thickness, has little effect by its own refractive 

 power, and serves only to give a proper form to. the aqueous humour, which 

 fills its concavity, and distends it. This humour is partially divided by the 

 uvea or iris, which is of different colours in different persons, having a perfora- 

 tion in its centre, called the pupil. Immediately behind the uvea, and closely 

 connected, to its base, are the ciliary processes, the summits of which hang,. 



